Method and apparatus for reducing jitter and improving testability of an oscillator

ABSTRACT

An oscillator such as a pulled-crystal oscillator (50) provides low clock jitter by converting a sinusoidal voltage on a crystal&#39;s (51) terminals into a digital square wave with a comparator (56). The oscillating frequency of the crystal (51) is pulled by selectively switching in extra capacitance through capacitor digital-to-analog converters (CDACs) (57, 58). The oscillator (50) has built-in testability which allows individual capacitors in the CDACs (57, 58) to be quickly tested for opens. A scan path is connected to the inputs of the CDACs (57, 58) for selecting individual capacitors. A first input terminal of the comparator (56) is precharged before a capacitor under test (171) is connected. A comparison voltage is provided to the second input terminal. The capacitor under test (171) is determined to be functional if, after being connected to the first input terminal of the comparator (56), it discharges the first input terminal to a voltage below the comparison voltage, causing the comparator (56) to switch.

CROSS-REFERENCE TO RELATED APPLICATION

Related subject matter is contained in my copending application, Ser. No. 08/111,792, filed concurrently herewith, entitled "Low Power Inverter for Crystal Oscillator Buffer or the Like" and assigned to the assignee hereof.

FIELD OF THE INVENTION

This invention relates generally to electrical circuits, and more particularly, to oscillators and testing methods and circuits therefor.

BACKGROUND OF THE INVENTION

A basic phase locked loop (PLL) has a phase detector which provides a phase detect output signal indicative of the phase difference between a loop clock signal and a reference clock signal. The phase detector provides the phase detect output signal to an input of a loop filter. The loop filter is a lowpass filter which provides an output voltage level indicative of the length of time the phase detector detects the two clock signals are out of phase. The output of the loop filter drives an input of a voltage controlled oscillator (VCO). The VCO then provides a clock output signal having a desired frequency. The clock output signal is divided in a loop divider to provide the loop clock signal. Thus, the PLL is able to generate the clock output signal having a frequency that is many times greater than that of the reference clock signal, based on the value of the loop divider.

A digital PLL uses an oscillator which responds to a digital code provided by a digital loop filter. This oscillator is variously referred to in the art as a VCO, a digital VCO (DVCO), a digitally-controlled oscillator (DXCO), or the like, although the term VCO will be used herein. One type of VCO used in digital PLLs is known as a pulled-crystal VCO. The pulled-crystal VCO is useful in applications in which the nominal clock output frequency is fixed but the actual frequency may vary by a relatively small amount. One case in which a pulled-crystal VCO is useful is a telecommunications system in which the far end provides data at a nominal rate which may vary by a certain amount, but does not provide a separate clock signal.

The pulled-crystal VCO uses not only a conventional resistor and inverter to bias a crystal, but also a variable capacitance. The pulled-crystal VCO changes the amount of load capacitance on the crystal's terminals to "pull" the frequency by relatively-small amounts. A capacitor digital-to-analog converter (CDAC) network provides the variable capacitance based on the loop filter's digital code. However, known CDACs present some problems, including testability, clock output signal jitter, and circuit design.

For proper testability, it is desirable to determine whether the CDAC's capacitors have been fabricated properly. Fabrication problems may result in either open circuits into one of the capacitor's terminals, or short circuits between the capacitor's terminals. In the CDAC section of the pulled-crystal VCO, short circuits may be easily tested. A test code connects all capacitors to the crystal terminals, and a testing apparatus detects a short if the amount of leakage current is excessive. Open circuit testing, however, is normally considered impractical. With a functional test it is possible to detect open capacitors, but this test requires a very long time for the clock output signal to stabilize to detect the small changes in output frequency. For example, CDAC capacitors may affect the clock output signal frequency by as little as fifteen parts-per-million (ppm). Thus the functional test must be run for enough cycles to detect a deviation from a frequency change of 15 ppm. Furthermore, each capacitor must be individually tested, multiplying this test period by the number of capacitors in the CDAC.

Another problem is clock output signal jitter. Switching additional capacitors onto crystal terminals results in a "glitch", which can introduce jitter into the clock output signal. One known method to minimize the effects of switching capacitors into the array was taught by Imamaru in U.S. Pat. No. 5,117,206, issued on May 26, 1992. Imamaru slowly changes the impedance of switching elements in order to avoid the effects of discretely switching a new capacitor into the circuit. However, Imamaru's capacitor array requires additional circuitry to generate the voltages provided to the switching elements.

The requirements for high performance and low power consumption also make circuit design of the crystal's inverter difficult. The inverter must be strong enough to drive a widely-varying capacitive load without switching transients, which cause signal jitter. The inverter must have matched pullup and pulldown characteristics in order to maintain proper duty cycle. However, the stronger the inverter, the higher the power consumption. Also, the pulled-crystal oscillator outputs a sinusoidal waveform which must be converted into a digital clock, further complicating circuit design.

Each of these problems exists to varying degrees in known pulled-crystal VCOs. It would thus be desirable to have an oscillator which simultaneously alleviates all these problems.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides, in one form, a method for reducing jitter and improving testability of an oscillator. One of a first plurality of capacitors is selected in response to a first digital input code. The selected one of the first plurality of capacitors is coupled between a first crystal terminal in response to a parallel clock signal and a predetermined voltage terminal. A capacitor is coupled between a second crystal terminal and the predetermined voltage terminal. The first crystal terminal is coupled to a first input terminal of a comparator. The second crystal terminal is coupled to a second input terminal of the comparator. A clock output signal is provided at an output terminal of the comparator. The parallel clock signal is generated synchronously to the clock output signal.

In still another form, the present invention provides an oscillator with low jitter, including first and second nodes, first and second capacitors, and a comparator. The first node is for being coupled to a first terminal of a crystal. The second node is for being coupled to a second terminal of the crystal. The first capacitor is coupled to the first node. The first capacitor comprises a first capacitor digital-to-analog converter (CDAC) switched by a first digital input code in response to a parallel clock signal. The second capacitor is coupled to the second node. The comparator has a first input terminal coupled to the first node, a second input terminal coupled to the second node, and an output terminal for providing a clock output signal. The parallel clock is characterized as being synchronous with the clock output signal.

These and other features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in partial logic diagram and partial schematic diagram form a pulled-crystal oscillator according to the prior art.

FIG. 2 illustrates in partial block diagram, partial logic diagram, and partial schematic diagram form a pulled-crystal oscillator according to the present invention.

FIG. 3 illustrates in partial block diagram and partial schematic diagram form one of the CDACs of FIG. 2.

FIG. 4 illustrates in schematic form one of the TEST CDACs of FIG. 2.

FIG. 5 illustrates partial block diagram, partial logic diagram, and partial schematic diagram form a portion of the pulled-crystal oscillator of FIG. 2 useful in understanding a test mode thereof.

FIG. 6 illustrates a timing diagram of the pulled-crystal oscillator of FIG. 2 during the test mode.

FIG. 7 illustrates in partial logic diagram and partial schematic diagram form a portion of the pulled-crystal oscillator of FIG. 2 useful understanding an advantage thereof in normal operation mode.

FIG. 8 illustrates in schematic form a crystal oscillator using an inverter according to the prior art.

FIG. 9 illustrates in schematic form the inverter of FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates in partial logic diagram and partial schematic diagram form a pulled-crystal oscillator 20 according to the prior art. Oscillator 20 includes generally a crystal 21, a resistor 22, an inverter 23, a first capacitor array 24, a second capacitor array 25, and a coupling circuit 26. Crystal 21 has first and second terminals. Resistor 22 has a first terminal connected to the first terminal of crystal 21, and a second terminal connected to the second terminal of crystal 21. Inverter 23 has a first terminal connected to the first terminal of crystal 21, and a second terminal connected to the second terminal of crystal 21.

Capacitor array 24 is connected to the first terminal of crystal 21 and includes N capacitors switched by a corresponding one of input signals labelled "D₀ -D_(N-1) 38 . Each leg of capacitor array 24 includes a capacitor and a switch, in series between the first terminal of crystal 21 and a power supply voltage terminal labelled "V_(SS) ". V_(SS) is a more-negative power supply voltage terminal having a nominal potential of approximately zero volts. An illustrative leg is shown including a capacitor 30, and a switch 31. Capacitor 30 has a first terminal connected to the first terminal of crystal 21, and a second terminal. Switch 31 has a first terminal connected to the second terminal of capacitor 30, and a second terminal connected to V_(SS), and is opened and closed by a digital control signal labelled "D_(i) ", where i corresponds to the significance of the capacitor. Capacitor array 24 also includes a capacitor 32 having a first terminal connected to the first terminal of crystal 21, and a second terminal connected to V_(SS).

Capacitor array 25 is connected to the second terminal of crystal 21 and includes N capacitors switched by a corresponding one of input signals D₀ -D_(N-1). Like array 24, each leg of capacitor array 25 includes a capacitor and a switch, in series between the second terminal of crystal 21 and V_(SS). An illustrative leg is shown including a capacitor 34, and a switch 35. Capacitor 34 has a first terminal connected to the second terminal of crystal 21, and a second terminal. Switch 35 has a first terminal connected to the second terminal of capacitor 32, and a second terminal connected to V_(SS), and is opened and closed by signal D_(i). Capacitor array 25 also includes a capacitor 36 having a first terminal connected to the second terminal of crystal 21, and a second terminal connected to V_(SS).

Coupling circuit 26 includes an inverter 40, a resistor 41, a capacitor 42, and inverters 43 and 44. Inverter 40 has an input terminal, and an output terminal connected thereto. Resistor 41 has a first terminal connected to the output terminal of inverter 40, and a second terminal. Capacitor 42 has a first terminal connected to the second terminal of crystal 21, and a second terminal connected to the second terminal of resistor 41. Inverter 43 has an input terminal connected to the second terminals of resistor 41 and capacitor 42, and an output terminal. Inverter 44 has an input terminal connected to the output terminal of inverter 43, and an output terminal for providing a signal labelled "CLOCK OUTPUT SIGNAL".

Oscillator 20 is a conventional pulled-crystal oscillator which has the problems of open capacitor testing, CLOCK OUTPUT SIGNAL jitter, and circuit design as mentioned previously. In particular, coupling circuit 26 converts a sinusoidal waveform into a clean, digital square wave. The circuit design choices used for coupling circuit 26 affect power consumption and duty cycle distortion. Coupling circuit 26 includes self-biased complementary metal-oxide-semiconductor (CMOS) inverter 40 in order to keep the input of inverter 43 near its switchpoint for fast switching. This self-biasing increases power consumption in inverter 40 because the P- and N-channel transistors are always somewhat conductive. Furthermore, in order to reduce power, it is desirable to make inverter 40 weak; however, if inverter 40 is made too weak, it no longer matches the threshold characteristics of inverter 43. The result is that the input of inverter 43 is biased above or below its switchpoint, resulting in clock duty cycle distortion. Coupling circuit 26 is also sensitive to power supply noise and noise that is alternating current (AC) coupled through capacitor 42.

FIG. 2 illustrates in partial schematic diagram, partial logic diagram, and partial block diagram form a pulled-crystal oscillator 50 according to the present invention. Pulled-crystal oscillator 50 includes generally a crystal 51, a resistor 52, an inverter 53, multiplexers 54 and 55 each labelled "MUX", a comparator 56, a first capacitor digital-to-analog converter (CDAC) labelled "CDAC A" 57, a second CDAC labelled "CDAC B" 58, a decoder 59, and a testing circuit 60. Crystal 51 has a first terminal labelled "XTAL_(IN) ", and a second terminal labelled "XTAL_(OUT) ". Resistor 52 has a first terminal connected to XTAL_(IN), and a second terminal connected to XTAL_(OUT). Inverter 53 has an input terminal connected to XTAL_(IN), and an output terminal connected to XTAL_(OUT). MUX 54 has a first input terminal connected to a node labelled "TEST A", a second input terminal connected to XTAL_(IN), a control input terminal for receiving a signal labelled "TEST MODE", and an output terminal. MUX 55 has a first input terminal connected to a node labelled "TEST B", a second input terminal connected to XTAL_(OUT), a control input terminal for receiving signal TEST MODE, and an output terminal. Comparator 56 has a negative input terminal connected to the output terminal of MUX 54, a positive input terminal connected to the output terminal of MUX 55, and an output terminal for providing CLOCK OUTPUT SIGNAL.

CDAC 57 is connected to XTAL_(IN), to the first terminal of MUX 54 at node TEST A, and to V_(SS), and receives an N-bit digital code labelled "DA₀ -DA_(N-1) " at an input terminal thereof and a signal labelled "SELECT A" at a control input terminal thereof. CDAC 58 is connected to XTAL_(OUT), to the second terminal of MUX 55 at node TEST B, and to V_(SS), and receives an N-bit digital code labelled "DB₀ -DB_(N-1) " at an input terminal thereof and a signal labelled "SELECT B" at a control input terminal thereof. The differences between CDACs 57 or 58 and capacitor arrays 24 or 25 of FIG. 1 are understood by referring now to FIG. 3, which illustrates in partial block diagram and partial schematic diagram form CDAC 57. CDAC 57 includes N capacitors each selectively switching a terminal of the corresponding capacitor between V_(SS) and TEST A. For illustrative purposes, CDAC 57 is described with reference to the i-th leg, which includes a capacitor 80, N-channel transistors 81 and 82, and a latch 83. Capacitor 80 has a first terminal connected to XTAL_(IN), and a second terminal Transistor 81 has a drain connected to the second terminal of capacitor 80, a gate for receiving a signal labelled "S_(i) ", and a source connected to V_(SS). Transistor 82 has a drain connected to the second terminal of capacitor 80, a gate for:receiving a signal labelled "T_(i) ", and a source connected to node TEST A. Latch 83 has a data input terminal for receiving signal DA_(i), a control input terminal for receiving signal SELECT A, a true output terminal for providing signal S_(i), and a gated complementary output terminal for providing signal T_(i).

CDAC 57 also includes a capacitor 85 and a resistor 86. Capacitor 85 has a first terminal connected to XTAL_(IN), and a second terminal connected to V_(SS). Capacitor 85 is a default capacitance which is always present and sets the basic frequency from which the frequency of oscillation is "pulled". Resistor 86 has a first terminal connected to node TEST A, and a second terminal connected to V_(SS). Resistor 86 is a high-valued resistor implemented as either a diffused resistor, or preferably a long-channel MOS transistor which may be made nonconductive when not needed. The advantage of resistor 86 will be described subsequently with reference to FIG. 7.

Signal SELECT A is active either during normal operation or when CDAC 57 is being tested. When signal SELECT A is inactive at a logic low, latch 83 functions to provide signal Si in response to signal DA_(i), and keeps signal T_(i) in an inactive state at a logic low. Thus, capacitor 80 will be selectively connected to V_(SS) through transistor 81, based on the value of corresponding input DA_(i). When signal SELECT A is active at a logic high, latch 83 provides signals S_(i) and T_(i) as complementary output signals. Thus, if DA_(i) =0, then signal S_(i) is inactive at a logic low keeping transistor 81 nonconductive, and signal T_(i) is active at a logic high to make transistor 82 conductive and to connect the second terminal of capacitor 80 to node TEST A. If DA_(i) =1, then signal S_(i) is active at a logic high making transistor 81 conductive, coupling capacitor 80 between XTAL_(IN) and V_(SS). Signal T_(i) is inactive at a logic low to keep transistor 82 nonconductive. Latch 83 makes signals S_(i) and T_(i) nonoverlapping with respect to each other in order to avoid momentary leakage from the first input terminal of MUX 54 through transistors 82 and 81 to V_(SS). Note that CDAC 58 is structurally identical to switched capacitor array 57 and in the following discussion, corresponding elements in CDAC 58 will be described by using the same reference number.

Returning now to FIG. 2, decoder 59 has an input terminal for receiving an m-bit digital input signal from a digital loop filter (not shown), and an output terminal for providing N-bit digital code D₀ -D_(N-1). Testing circuit 60 includes a CDAC 61 labelled "TEST CDAC B", a CDAC 62 labelled "TEST CDAC A", and a scan path formed by scannable data latches 63-65. TEST CDAC 61 is connected to node TEST A, and receives signal TEST MODE, a signal labelled "TEST ENABLE B", and a signal labelled "SCAN ENABLE" at control input terminals thereof. TEST CDAC 62 is connected to node TEST B, and receives signals TEST MODE and SCAN ENABLE and a signal labelled "TEST ENABLE A" at control input terminals thereof. TEST CDACS 61 and 62 are understood with reference to FIG. 4, which illustrates in schematic form TEST CDAC 61 of FIG. 2.

TEST CDAC 61 includes a P-channel transistor 90, a capacitor 91, N-channel transistors 93-95, and a capacitor 96. Transistor 90 has a source connected to a power supply voltage terminal labelled "V_(DD) ", a gate for receiving a complement of signal SCAN ENABLE labelled "SCAN ENABLE", and a drain connected to TEST A. V_(DD) is a more-positive power supply voltage terminal representative of a logic high voltage and having a nominal voltage of approximately 5.0 volts. Signal SCAN ENABLE is preferably generated globally by inverting signal SCAN ENABLE and providing the resulting signal to each of TEST CDACs 61 and 62, but this function was omitted from FIG. 2 to ease discussion. Capacitor 91 has a first terminal connected to the drain of transistor 90, and a second terminal. Transistor 93 has a drain connected to the second terminal of capacitor 91, a gate for receiving signal TEST MODE, and a source connected to V_(SS). Transistor 94 has a drain connected to the drain of transistor 90, a gate for receiving signal TEST ENABLE B, and a source. Transistor 95 has a drain connected to the source of transistor 94, a gate for receiving signal SCAN ENABLE, and a source connected to V_(SS). Capacitor 96 has a first terminal connected to the source of transistor 94, and a second terminal connected to V_(SS).

When VCO 50 is in normal operation mode, signals TEST MODE, SCAN ENABLE, SCAN ENABLE, and TEST ENABLE B are inactive at their respective logic levels. During normal operation mode, transistors 90, 93, 94, and 95 are inactive and capacitors 91 and 96 are floating. During test mode, signal TEST MODE is active at a logic high, making transistor 93 conductive to connect the second terminal of capacitor 91 to V_(SS). When test data is being shifted into the scan path, signal SCAN ENABLE becomes active, precharging the first terminal of capacitor 91 to V_(DD). Signal SCAN ENABLE also becomes active, discharging the first terminal of capacitor 96 to V_(SS). Signal TEST ENABLE B remains inactive, isolating the first terminal of capacitor 96 from node TEST A. When the scan data has been properly shifted into the scan path, signals SCAN ENABLE and SCAN ENABLE again become inactive for testing, but by this time the first terminal of capacitor 91 has been precharged to approximately V_(DD).

During the actual test, signal TEST ENABLE B becomes active, connecting the first terminal of capacitor 96 to node TEST A, and causing capacitor 91 to discharge into capacitor 96. In the preferred embodiment, capacitors 91 and 96 are equal-valued. In the absence of any parasitic capacitance, the voltage on node TEST A should approach approximately V_(DD) /2 or about 2.5 volts. However, the parasitic capacitance on node TEST A also stores charge during the precharge period, increasing the capacitance with which capacitor 96 shares charge. Thus, the voltage on node TEST A will be above V_(DD) /2. This voltage is provided through MUX 54 of FIG. 2 to the negative input terminal of comparator 56 to provide a comparison voltage, or offset, against which CDAC 58 is tested. The structure and operation of TEST CDAC 62, which is used when testing CDAC 57, is identical except that TEST CDAC 62 is connected to node TEST B, and a transistor corresponding to transistor 94 receives signal TEST ENABLE A at its gate.

Returning now to FIG. 2, scannable data latch 63 has a parallel input terminal connected to the output terminal of decoder 59 for receiving signals D₀ -D_(N-1), a parallel clock input terminal for receiving a signal labelled "PARALLEL CLOCK", a serial data input terminal for receiving a signal labelled "SCAN DATA IN", a scan clock input terminal for receiving a signal labelled "SCAN CLOCK", a control input terminal for receiving signal SCAN ENABLE, a parallel output terminal for providing signals DA₀ -DA_(N-1) to capacitor array 54, and a serial data output terminal. Signal PARALLEL CLOCK is a high-speed clock which is some sub-multiple of CLOCK OUTPUT SIGNAL and which switches synchronously therewith. In the illustrated embodiment, PARALLEL CLOCK has a frequency equal to one-half the frequency of CLOCK OUTPUT SIGNAL.

Scannable data latch 64 has a parallel input terminal connected to the output terminal of comparator 56, a serial data input terminal connected to the serial data output terminal of scannable data latch 63, a scan clock input for receiving signal SCAN CLOCK, a control input terminal for receiving signal SCAN ENABLE, and a serial data output terminal. Scannable data latch 65 has a parallel input terminal connected to the output terminal of decoder 59 for receiving signals D₀ -D_(N-1), a parallel clock input terminal for receiving signal PARALLEL CLOCK, a serial data input terminal connected to the serial data output terminal of scannable data latch 64, a scan clock input signal for receiving signal SCAN CLOCK, a control input terminal for receiving signal SCAN ENABLE, a parallel output terminal for providing signals DB₀ -DB_(N-1) to capacitor array 58, and a serial data output terminal for providing a signal labelled "SCAN DATA OUT".

During normal operation mode, signal SCAN ENABLE is inactive, and scannable data latches 63 and 65 latch new values of input signals D₀ -D_(N-1) to provide as output signals DA₀ -DA_(N-1) and DB₀ -DB_(N-1), respectively synchronously with the PARALLEL CLOCK. During test mode, MUXes 54 and 55 operatively disconnect crystal 51 from comparator 56, stopping the CLOCK OUTPUT SIGNAL and the PARALLEL CLOCK from switching. Thus, during test mode, scannable data latches 63 and 65 no longer latch new values of parallel input data signals D₀ -D_(N-1). The SCAN DATA IN signal is used to shift serial test data in synchronously with the SCAN CLOCK. Signal SCAN ENABLE is active while data is being shifted in, inhibiting the output of latches 63 and 65. Once the test data is properly aligned in scannable data latches 63 and 65, signal SCAN ENABLE is deactivated to enable the outputs thereof.

In overall operation, pulled-crystal oscillator 50 has two modes of operation, normal operation mode and test mode. During normal operation mode, pulled-crystal oscillator 50 provides low signal jitter and avoids difficult circuit design tradeoffs, resulting in both low power consumption and a reliable fifty percent duty cycle clock. In normal operation mode, various capacitors in CDACs 57 and 58 are switched into oscillator 50 by digital codes DA₀ -DA_(N-1) and DB₀ -DB_(N-1), respectively, each of which is the same as digital code D₀ - D_(N-1). In other embodiments, digital code DA₀ -DA_(N-1) need not be the same as digital code DB₀ -DB_(N-1). In test mode, pulled-crystal oscillator 50 provides a scannable testing apparatus which is able to detect open capacitors in a short amount of time. In test mode, digital codes DA₀ -DA_(N-1) and DB₀ -DB_(N-1) enable selected capacitors in response to a sequence of data bits conducted through latches 63-65 through the SCAN DATA IN signal line and in general, DA₀ -DA_(N-1) is not the same as DB₀ -DB_(N-1) during test mode.

Normal operation mode is signified by signal TEST MODE being inactive at a logic low. In normal operation mode, testing circuit 60 is disabled, CDACs 54 and 55 are enabled; and MUXes 54 and 55 select the second inputs thereof. In normal operation mode, pulled-crystal oscillator 50 consumes less power than oscillator 20 of FIG. 1 because the inputs of comparator 56 do not need to be biased. Oscillator 50 also provides CLOCK OUTPUT SIGNAL with less signal jitter than oscillator 20 because comparator 56 is a comparator with high common-mode and power-supply rejection, and thus is not susceptible to being biased above or below its switchpoint or to noise-induced changes in its switchpoint.

Test mode is signified by signal TEST MODE being active at a logic high. FIG. 5 illustrates partial block diagram, partial logic diagram, and partial schematic diagram form a portion 150 of pulled-crystal oscillator 50 of FIG. 2 useful in understanding a test mode thereof. Pulled-crystal oscillator 150 includes generally portions of MUXes 54 and 55, comparator 56, CDAC 57, TEST CDAC 62, and scannable latches 63 and 64. It should be understood that while the same reference numbers are used for corresponding elements of FIG. 2, some of the circuitry used has been omitted or simplified.

FIG. 5 illustrates an "A" testing cycle in which capacitors in CDAC 57 are tested. Thus, CDAC 57 and TEST CDAC 62 are operatively connected to comparator 56. MUX 54 disconnects XTAL_(IN) from the negative input terminal of comparator 56, represented by a switch 151 thereof being open. Another switch 155 is closed to connect XTAL_(IN), to V_(SS). MUX 54 connects node TEST A to the negative input terminal of comparator 56, represented by a switch 153 thereof being closed. Likewise, MUX 55 disconnects XTAL_(OUT) from the positive input terminal of comparator 56, represented by a switch 161 thereof being open. Another switch 165 is closed to connect XTAL_(OUT) to V_(SS). MUX 55 connects node TEST B to the positive input terminal of comparator 56, represented by a switch 163 thereof being closed.

FIG. 5 shows two illustrative legs of CDAC 57, including a most significant leg with a capacitor 171, N-channel transistors 172 and 173, and an inverter 174. Capacitor 171 has a first terminal connected to XTAL_(IN), and a second terminal. Transistor 172 has a drain connected to the second terminal of capacitor 171, a gate for receiving signal DA_(N-1), and a source connected to V_(SS). Transistor 173 has a drain connected to node TEST A, a gate, and a source connected to the drain of transistor 172. Inverter 174 has an input terminal for receiving signal DA_(N-1), and an output terminal connected to the gate of transistor 173.

A capacitor is tested by setting a corresponding scan data bit to a binary zero, and setting all other scan data bits to binary ones. Signal SCAN ENABLE is activated to scan this data in through the SCAN DATA IN signal line synchronously with SCAN CLOCK. Scannable data latch 63 includes N D-type scan flip flops including illustrative flip flops 180 and 181. To test capacitor 171, a binary zero is scanned in through SCAN DATA IN during a first clock, followed by N-1 binary ones. After these N cycles of SCAN CLOCK, the binary zero has propagated into scan flip flop 181. Thus, the Q output terminal of scan flip flop 181 is set to a binary zero, making transistor 172 nonconductive and transistor 173 conductive. The result is that the first terminal of capacitor 171 is connected to V_(SS), through switch 155 in MUX 54, and the second terminal of capacitor 171 is connected to the negative input terminal of comparator 56 through node TEST A.

During data input scanning, signal SCAN ENABLE causes transistors 90 of TEST CDACs 61 and 62 to precharge nodes TEST A and TEST B, respectively, to V_(DD). These voltages are held at approximately V_(DD) by a parasitic capacitance 175 on the negative input terminal of comparator 56, and by a parasitic capacitance 176 on the positive input terminal of comparator 56 and by capacitors 91 in TEST CDACs 61 and 62, respectively. After the data has been aligned in scannable data latch 63, signal SCAN ENABLE becomes inactive and transistors 90 of TEST CDACs 61 and 62 are nonconductive. Capacitors 91 in TEST CDAC 62 and 176 discharge into capacitor 96, through a path represented by a switch 94, assigned the same reference number as transistor 94 of FIG. 4, being closed. The first terminal of capacitor 96 had previously been discharged to V_(SS), through a path represented now by a switch 95, assigned the same reference number as transistor 95 of FIG. 4, being open. Capacitors 91 and 96 preferably have equal values which are smaller than the smallest capacitor in CDAC 57. Thus, the comparison voltage on the positive input terminal of comparator 56 will be somewhat above mid-supply due to parasitic capacitor 176, and is preferably made about 3.75 volts.

This comparison voltage initially causes comparator 56 to switch low, because the 5.0 volts on the negative terminal of comparator 56 is greater than the 3.75 volts on the positive terminal thereof. However, if capacitor 171 and the associated control circuitry (transistors 172 and 173 and inverter 174) are functional, capacitors 91 and 175 will discharge into capacitor 171, bringing the negative terminal of comparator 56 to a lower voltage than the comparison voltage. This discharging is guaranteed by sizing capacitor 96 to be smaller than the smallest capacitor in CDAC 57 and having approximately equal parasitic capacitances 175 and 176.

FIG. 6 illustrates a timing diagram of pulled-crystal oscillator 50 of FIG. 2 during the test mode. FIG. 6 illustrates an "A" testing cycle, in which a selected capacitor in CDAC 57 is tested. FIG. 6 illustrates four signals, SCAN CLOCK, SCAN ENABLE, TEST ENABLE A, and the output of comparator 56, versus time on the horizontal axis. FIG. 6 also illustrates four intervals relevant to the test sequence. A first interval, labelled "SCAN IN", occurs when data is being read into the scan path. During the SCAN IN interval, signal SCAN ENABLE is active, inhibiting the outputs of scannable data latches 63 and 65 of FIG. 2, and discharging capacitor 96 of FIG. 4 to V_(SS). Active signal SCAN ENABLE (not shown in FIG. 6) also causes the input signal lines to comparator 56 to be precharged to approximately V_(DD) through transistors 90. Signal TEST ENABLE A is inactive, isolating capacitor 96 from node TEST A. Data is input serially and shifted through the scan path synchronously with SCAN CLOCK.

A second interval, labelled "TEST", follows the SCAN IN interval. During the TEST interval, signal SCAN ENABLE becomes inactive, causing scannable data latches 63 and 65 to provide their outputs to CDACs 57 and 58, respectively. Signal TEST ENABLE A becomes active, providing a comparison voltage to the positive input terminal of comparator 56 by causing the charge stored in capacitors 91 and 176 to discharge into capacitor 96. The connection of the selected capacitor in CDAC 57 causes the output of comparator 56 to reflect the outcome of the test during the TEST interval.

During a third interval, labelled "LATCH", the output of comparator 56 is latched into the scan path. Scannable data latch 64 latches the output of comparator 56 on the falling edge of SCAN CLOCK.

A fourth interval labelled "SCAN OUT" follows the LATCH interval. During the SCAN OUT interval, signal SCAN ENABLE is active, again inhibiting the outputs of scannable data latches 63 and 65. Data is output serially and shifted through the scan path synchronously with SCAN CLOCK until the data bit corresponding to the output of comparator 56 is output as signal SCAN DATA OUT. Preferably, a single physical latch implements the function of latches 63 and 65. This sharing of circuitry is possible because during the normal operation mode, corresponding capacitors in CDACs 57 and 58 are activated simultaneously, and during test mode signals SELECT A and SELECT B ensure that only one of CDACs 57 and 58 are tested at any one time. Thus, a single clock is required during the SCAN OUT interval to output the value of comparator 56 latched during the LATCH interval as signal SCAN DATA OUT, keeping test time to a minimum.

In a preferred test sequence, testing of capacitors in CDACs 57 and 58 is preceded by a pretest sequence which tests "A" and "B" control circuitry. For example, in order to test the "A" control circuitry, a sequence of N binary ones is scanned into scannable data latch 63. Since no capacitors in CDAC 57 are connected, capacitor 96 in TEST CDAC 62 should pull the voltage at the positive input terminal of comparator 56 below the voltage at the negative input terminal, causing comparator 56 to output a logic low.

The "A" capacitors are tested one at a time by setting signal SELECT A to a logic high and signal SELECT B to a logic low. Each capacitor is tested in turn by scanning in a binary zero in the corresponding bit position zero, and a binary one in all other bit positions. In the test of CDAC 57, if the i-th capacitor is not open, then it will pull the voltage at the negative input terminal of comparator 56 below that at the positive input terminal thereof. Thus, if the output of comparator 56 is a logic high, then the i-th capacitor is not open. The test of the "B" capacitors in CDAC 58 proceeds in a similar fashion, except that a logic low output of comparator 56 indicates a functional capacitor. It is possible to write control software to either test all elements, or to exit the test after the first defective element is detected. Note that while it is preferred to pretest control circuitry, other test sequences may test both the control circuitry and the CDAC capacitors together.

FIG. 7 illustrates in partial logic diagram and partial schematic diagram form a portion 250 of pulled-crystal oscillator 50 of FIG. 2 useful in understanding an advantage thereof in normal operation mode. In pulled-crystal oscillator 250, crystal 51, resistor 52, inverter 53, and comparator 56 are connected as in FIG. 2. Note that MUXes 54 and 55 are omitted for clarity. CDAC 57 includes capacitor 85, an active capacitor 251, and N-1 inactive capacitors 252. Since it is active, capacitor 251 has a first terminal connected to XTAL_(IN), and a second terminal connected to V_(SS). Since signal SELECT A is active, each of the N-1 inactive capacitors has a first terminal connected to XTAL_(IN), and a second terminal connected to node TEST A. CDAC 58 includes capacitor 85, an active capacitor 253, and N-1 inactive capacitors 254. Since it is active, capacitor 253 has a first terminal connected to XTAL_(OUT), and a second terminal connected to V_(SS). Since signal SELECT B is active, each of the N-1 inactive capacitors has a first terminal connected to XTAL_(OUT), and a second terminal connected to node TEST B.

In normal operation mode, the second terminals of the inactive capacitors are connected to either TEST A or TEST B. Since CDACs 57 and 58 connect these nodes to V_(SS) through high-valued resistors 86, the inactive capacitors slowly charge up to the average voltage present on the XTAL_(IN) and XTAL_(OUT) nodes. The values of resistors 86 must be chosen such that the RC time constant, for a capacitance of the smallest capacitor in CDACs 57 and 58, is much longer than the period of oscillation. The result is that when an inactive capacitor is activated, it is already charged to the average voltage on a corresponding one of nodes XTAL_(IN) and XTAL_(OUT). The switchpoint occurs at the zero-crossing point of the inputs of comparator 56 because new input data is latched by PARALLEL CLOCK, and thus switches synchronously with CLOCK OUTPUT SIGNAL. Since the zero-crossing point occurs near the average voltages on the XTAL_(IN) and XTAL_(OUT) nodes, there will be virtually no "glitch" on XTAL_(IN) and XTAL_(OUT) when the inactive capacitors are switched in.

FIG. 8 illustrates in schematic form a crystal oscillator 300 using an inverter 305 according to the prior art. Crystal oscillator 300 includes a crystal 301, a resistor 302, capacitors 303 and 304, and inverter 305. Crystal 301 has first and second terminals. Resistor 302 has a first terminal connected to the first terminal of crystal 301, and a second terminal connected to the second terminal of crystal 301. Capacitor 303 has a first terminal connected to the first terminal of crystal 301, and a second terminal connected to V_(SS). Capacitor 304 has a first terminal connected to the second terminal of crystal 301, and a second terminal connected to V_(SS). Inverter 305 has a first terminal connected to the first terminal of crystal 301, and a second terminal connected to the second terminal of crystal 301. Inverter 305 is a conventional CMOS inverter including a P-channel transistor 306, and an N-channel transistor 307. Transistor 306 has a source connected to V_(DD), a gate connected to the first terminal of crystal 301, and a drain connected to the second terminal of crystal 301. Transistor 307 has a drain connected to the drain of transistor 306, a gate connected to the first terminal of crystal 301, and a source connected to V_(SS).

Oscillator 300, known as a Pierce oscillator, is commonly used in CMOS integrated circuits. Crystal 301 is an off-chip crystal having a desired resonant frequency. Resistor 302 and capacitors 303 and 304 are normally off-chip because they require too much integrated circuit area in most embodiments, but may be integrated in embodiments requiring relatively-small values or in pulled-crystal oscillators. Transistors 306 and 307 are almost always integrated on-chip using conventional CMOS processes.

Although oscillator 300 is simple and reliable, it has higher power consumption in applications in which V_(DD) is equal to approximately 5 volts and which require large values for capacitors 303 and 304, such as a pulled-crystal oscillator. Oscillator 300 is a good solution when V_(DD) is slightly above (V_(TN) +|V_(TP) |), where V_(TN) is the gate-to-source threshold voltage of transistor 307, and V_(TP) is the gate-to-source threshold voltage of transistor 306. However, when V_(DD) is about 5.0 volts and V_(TN) and |V_(TP) | are each about 0.8 volts, and transistors 306 and 307 in inverter 305 are sized strongly enough to oscillate with a large capacitive load, inverter 305 will have undesirably large overlap current. It is possible to provide a reduced positive power supply voltage at the source of transistor 306, but the reduced supply will require an additional package pin for a decoupling capacitor, which may not be available in some applications. In addition, it is always desirable to minimize integrated circuit pin count. Thus, a new inverter design is needed in order to reduce power consumption for such applications.

FIG. 9 illustrates in schematic form inverter 53 of FIG. 2. Inverter 53 includes generally an P-side source-follower portion 310, an N-side source-follower portion 320, a P-side gate control portion 330, an N-side gate control portion 340, a P-channel transistor 350, and an N-channel transistor 360.

P-side source-follower portion includes a current source 311, a P-channel transistor 312, and a current source 313. Current source 311 has a first terminal connected to V_(DD), and a second terminal, and conducts a current labelled "I₃₁₁ ". Transistor 312 has a source connected to the second terminal of current source 311, a gate for receiving an input voltage signal labelled "V_(IN) ", and a drain. In inverter 53, V_(IN) is connected to the XTAL_(IN) terminal of crystal 51, but in other embodiments may be an arbitrary input voltage such as a CMOS logic signal. Current source 313 has a first terminal connected to the drain of transistor 312, and a second terminal connected to V_(SS), and conducts a current labelled "I₃₁₃ ". Note that as used herein, all current conducting devices are considered to be current "sources", although current source 313 may also be considered a current sink.

N-side source-follower portion 320 includes a current source 321, a N-channel transistor 322, and a current source 323. Current source 321 has a first terminal connected to V_(DD), and a second terminal, and conducts a current labelled "I₃₂₁ ". Transistor 322 has a drain connected to the second terminal of current source 321, a gate for receiving signal V_(IN), and a source. Current source 323 has a first terminal connected to the source of transistor 322, and a second terminal connected to V_(SS), and conducts a current labelled "I₃₂₃ ".

P-side gate control portion 330 includes P-channel transistors 331 and 332. Transistor 331 has a source connected to V_(DD), a gate connected to the source of transistor 312, and a drain connected to the source of transistor 312. Transistor 332 has a source connected to V_(DD), a gate connected to the second terminal of current source 321, and a drain connected to the source of transistor 312. N-side gate control portion 340 includes a single N-channel transistor also referred to as 340. Transistor 340 has a drain connected to the source of transistor 322, a gate connected to the source of transistor 322, and a drain connected to V_(SS). P-channel transistor 350 has a source connected to V_(DD), a gate connected to the source of transistor 312, and a drain for providing a signal labelled "V_(OUT) ". In inverter 53, V_(OUT) is connected to the XTAL_(OUT) terminal of crystal 51, but in other embodiments may be considered a CMOS logic signal. N-channel transistor 360 has a drain connected to the drain of transistor 350, a gate connected to the source of transistor 322, and a source connected to V_(SS).

Inverter 53 is very effective in minimizing overlap currents. First, inverter 53 includes source follower transistor pairs 312 and 350, and 322 and 360. These source-follower pairs reduce the available headroom between V_(DD) and the sum of the thresholds of transistors 350 and 360. Second, transistor 331 in P-side gate control portion 330 and transistor 340 in N-side gate control portion 340 are clamping transistors which limit the gate-to-source voltages seen by output transistors 350 and 360, respectively. The current in P-side gate control portion 330 is limited to the difference in current sources 313 and 311, namely a value of (I₃₁₃ -I₃₁₁). Likewise, the current in N-side gate control portion 340 is limited to the difference in current sources 321 and 323, namely a value of (I₃₂₁ -I₃₂₃). Currents I₃₁₃ and I₃₂₁ must be greater than currents I₃₁₁ and I₃₂₃, respectively, and these differences and appropriate transistor size ratios determine the maximum currents flowing through transistors 350 and 360, respectively. Specifically, the maximum current through transistor 350 is equal to

    I.sub.350 (MAX)=(I.sub.313 -I.sub.311)*(W/L).sub.350 /(W/L).sub.331[ 1]

where I₃₅₀ (MAX) is the approximate maximum drain current of transistor 350, (W/L)₃₅₀ is the gate width-to-length ratio of transistor 350, and (W/L)₃₃₁ is the gate width-to-length ratio of transistor 331. Likewise, the maximum current through transistor 360 is equal to

    I.sub.360 (MAX)=(I.sub.321 -I.sub.323)*(W/L).sub.360 /(W/L).sub.340[ 2]

where I₃₆₀ (MAX) is the approximate maximum drain current of transistor 360, (W/L)₃₆₀ is the gate width-to-length ration of transistor 360, and (W/L)₃₄₀ is the gate width-to-length ratio of transistor 340.

As long as currents I₃₁₁, I₃₁₃, I₃₂₁, and I₃₂₃ are reasonably well-controlled, the output current will be relatively independent of changes in device characteristics due to process, temperature, and power supply variations. Current sources 311 and 321 are preferably formed by P-channel transistors biased by a stable bias voltage, and current sources 313 and 323 are preferably formed by N-channel transistors biased by another stable bias voltage. These bias voltages are preferably generated from a stable reference (not shown) such as from a V_(BE) /R or ΔV_(BE) /R generator. As used here, "V_(BE) " is the forward-biased base-to-emitter diode voltage drop of a reference bipolar transistor, "ΔV_(BE) " is the difference in V_(BE) between two bipolar transistors, and "R" is the resistance of a reference resistor. The bias voltages are preferably provided by a bandgap reference voltage generator, providing a ΔV_(BE) /R reference. Because current I is very stable, transistors 350 and 360 may be made strong (i.e., have large W/L ratios) without having large overlap currents. This strength is desirable when inverter 53 is used in crystal oscillator inverters because it ensures oscillation even with high capacitive loading.

Transistor 332 is a "helper" device that helps to make transistor 350 nonconductive when V_(IN) switches to a logic high. Being a P-channel device, transistor 350 will normally have a gate width-to-length ratio two to three times greater than that of transistor 360. However this gate sizing provides correspondingly greater capacitance on the source of transistor 312. When voltage V_(IN) is increasing, the combined effects of clamping transistor 331 and current source 311 may slow the increase in voltage at the gate of transistor 350 so much that a large overlap current in transistors 350 and 360 results. Under these conditions, the voltage at the drain of transistor 322 is falling; thus, helper transistor 332 biased by the voltage at the drain of transistor 322 provides additional current to make transistor 350 nonconductive quicker.

Note that a similar helper device cannot be used to help make transistor 360 nonconductive quicker, because having such a helper device would form a latch structure which could make both transistor 350 and transistor 360 simultaneously nonconductive. However, the smaller capacitance on the gate of transistor 360 reduces the need for a second helper device.

Inverter 53 has greatly reduced power consumption over inverter 300 of FIG. 8 both when V_(DD) is equal to the standard 5.0 volts and at a reduced value of 3.3 volts. Table I illustrates simulation results when inverters 300 and 53 were used in an oscillator with a 20.48 megahertz (MHz) crystal resonance frequency:

                  TABLE I                                                          ______________________________________                                                             Typical Power                                                                              Maximum Power                                  Circuit  V.sub.DD (V)                                                                              (mW)        (mW)                                           ______________________________________                                         Inverter 300                                                                            5.0        14.1        39                                             Inverter 300                                                                            3.3        9.0         25.6                                           Inverter 53                                                                             5.0        6.75        14.85                                          ______________________________________                                    

For the purposes of this comparison, "typical" means 25 degrees Celsius, typical processing parameters, and 50 picofarads (pF) on V_(IN) and V_(OUT) (connected to nodes XTAL_(IN) and XTAL_(OUT), respectively). "Maximum" means -40 degrees Celsius, best case speed processing (lowest N- and P-channel thresholds, shortest effective channel lengths, etc.), and 100 picofarads (pF) on V_(IN) and V_(OUT). In addition, for each of the three cases, the last-stage transistors were sized as small as possible while still ensuring oscillation under all operating and processing conditions. Thus, inverter 53 significantly reduces power consumption over inverter 300 at 5.0 volts. Inverter 53 also reduces power consumption even when inverter 300 is powered from a reduced-supply voltage at 3.3 volts.

While inverter 53 includes both P- and N-side source follower and clamp circuits, either the P- or the N-side circuitry may be used independently to reduce overlap current. Current sources 313 and 321, connected to the drains of their respective source follower transistors, may be omitted in other embodiments. However, current source 321 is important to the operation of helper transistor 332 and should be used when helper transistor 332 is present.

Referring to FIG. 5 in conjunction with FIG. 9, note that during test mode, both nodes XTAL_(IN) and XTAL_(OUT) are connected to V_(SS), potentially causing a short circuit through transistor 350 from V_(DD) to V_(SS). This short circuit may be avoided in a variety of ways, such as by disconnecting V_(IN) and V_(OUT) from nodes XTAL_(IN) and XTAL_(OUT), respectively, during test mode. Signal V_(OUT) also may be placed in a high impedance state by connecting the gates of transistors 350 and 360 to V_(DD) and V_(SS), respectively, during the test mode. Preferably, however, the gates of both transistors 350 and 360 are driven to V_(DD) during test mode so that transistor 360 performs the function of switch 165 to connect node XTAL_(OUT) to V_(SS).

While the invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume many embodiments other than those specifically set out and described above. For example, the present invention contemplates different test sequences, both with and without pretests. Other embodiments may use different control signals, and implement scannable data latches 63 and 65 of FIG. 2 in separate circuitry. Various forms of resistors such as diffused resistors, long-channel transistors, and the like are contemplated. Note that TEST CDACs 61 and 62 operate to introduce an appropriate comparison voltage at the input of comparator 56 against which the capacitors of CDACs 58 and 57 may be tested. While this method of providing the comparison voltage is preferred because variations in capacitor sizing in the TEST CDAC will track variations in the CDAC under test, alternate methods of providing the comparison voltage are possible. The alternate methods include but are not limited to: selectively creating an imbalance in current sources in an input stage of comparator 56; selectively creating an imbalance in input device sizes of a differential input stage of comparator 56; and connecting the appropriate input terminal of comparator 56 to a reference voltage. Also in other embodiments, one of CDACs 61 and 62 may be replaced with a fixed capacitor, and the oscillator frequency is "pulled" by changing the capacitance value at only one of nodes XTAL_(IN) and XTAL_(OUT). In addition, the comparator may be connected to crystal terminals of a fixed capacitor oscillator to reduce clock output signal jitter. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention. 

I claim:
 1. A method for reducing jitter and improving testability of an oscillator, comprising the steps of:selecting one of a first plurality of capacitors in response to a first digital input code; coupling said selected one of said first plurality of capacitors between a first crystal terminal and a predetermined voltage terminal in response to a parallel clock signal; coupling a capacitor between a second crystal terminal and said predetermined voltage terminal; coupling said first crystal terminal to a first input terminal of a comparator; coupling said second crystal terminal to a second input terminal of said comparator; providing a clock output signal at an output terminal of said comparator; generating said parallel clock signal synchronously with said clock output signal; and charging others of said first plurality of capacitors besides said selected one of said first plurality of capacitors to an average voltage on said first crystal terminal.
 2. The method of claim 1, wherein said step of charging comprises the steps of:coupling others of said first plurality of capacitors between said first crystal terminal and a reference terminal; and coupling a resistor between said reference terminal and said predetermined voltage terminal.
 3. An oscillator with low jitter, comprising:a first node for being coupled to a first terminal of a crystal; a second node for being coupled to a second terminal of said crystal; a first capacitor coupled to said first node, said first capacitor comprising a first capacitor digital-to-analog converter (CDAC) switched by a first digital input code in response to a parallel clock signal; a second capacitor coupled to said second node; a comparator having a first input terminal coupled to said first node, a second input terminal coupled to said second node, and an output terminal for providing a clock output signal; said parallel clock signal characterized as being synchronous with said clock output signal; a plurality of capacitors switched by said first digital input code; a selected one of said plurality of capacitors coupled between said first node and a predetermined voltage terminal; others besides said selected one of said plurality of capacitors having a first terminal coupled to said first node, and a second terminal coupled to a reference terminal; and a resistor having a first terminal coupled to said reference terminal, and a second terminal coupled to said predetermined voltage terminal.
 4. The oscillator of claim 3 wherein a resistance of said resistor is sufficient to make a resistance-capacitance (RC) time constant of said resistor and a smallest capacitor of said first CDAC greater than a period of said clock output signal.
 5. A method for reducing jitter and improving testability of an oscillator, comprising the steps of:selecting one of a first plurality of capacitors comprising a first capacitor Digital-to-Analog (CDAC) in response to a first digital input code; coupling said selected one of said first plurality of capacitors between a first crystal terminal and a predetermined voltage terminal; charging others of said first plurality of capacitors besides said selected one of said first plurality of capacitors to an average voltage on said first crystal terminal; coupling a capacitor between a second crystal terminal and said predetermined voltage terminal; coupling said first crystal terminal to a first input terminal of a comparator; coupling said second crystal terminal to a second input terminal of said comparator; providing a clock output signal at an output terminal of said comparator; and changing said first digital input code synchronously with said clock output signal.
 6. The method of claim 5, wherein said step of charging comprises the steps of:coupling others of said first plurality of capacitors between said first crystal terminal and a reference terminal; and coupling a resistor between said reference terminal and said predetermined voltage terminal.
 7. The method of claim 6 wherein said step of coupling said resistor comprises the step of coupling a resistor, having a resistance sufficient to make a resistance-capacitance (RC) time constant of said resistor and a smallest capacitor of said first CDAC greater than a frequency of said clock output signal, between said reference terminal and said predetermined voltage terminal.
 8. The method of claim 5 wherein said step of coupling said capacitor comprises the steps of:selecting one of a second plurality of capacitors in response to a second digital input code; coupling said selected one of said second plurality of capacitors between said second crystal terminal and said predetermined voltage terminal; and charging others of said second plurality of capacitors besides said selected one of said second plurality of capacitors to an average voltage on said second crystal terminal.
 9. An oscillator with low jitter, comprising:a first node for being coupled to a first terminal of a crystal; a second node for being coupled to a second terminal of said crystal; a first plurality of capacitors switched by a first digital input code in response to parallel clock signal; a selected one of said first plurality of capacitors coupled between said first node and a predetermined voltage terminal; another one of said first plurality of capacitors besides said selected one of said first plurality of capacitors having a first terminal coupled to said first node, and a second terminal coupled to a reference terminal; first charging means coupled to said reference terminal, for charging said other one of said first plurality of capacitors to an average voltage of said first node; a second capacitor coupled to said second node; and a comparator having a first input terminal coupled to said first node, a second input terminal coupled to said second node, and an output terminal for providing a dock output signal; said parallel clock signal characterized as being synchronous with said clock output signal.
 10. The oscillator of claim 9 wherein said charging means comprises a resistor having a first terminal coupled to said reference terminal, and a second terminal coupled to said predetermined voltage terminal.
 11. The oscillator of claim 10 wherein a resistance of said resistor is sufficient to make a resistance-capacitance (RC) time constant of said resistor and a smallest capacitor of said first plurality of capacitors greater than a period of said clock output signal.
 12. The oscillator of claim 9 further comprising:a second plurality of capacitors switched by a second digital input code; a selected one of said second plurality of capacitors coupled between said node and said predetermined voltage terminal; another one of said second plurality of capacitors besides said selected one of said second plurality of capacitors having a first terminal coupled to said second node, and a second terminal coupled to said reference terminal; and second charging means coupled to said reference terminal, for charging said other one of said second plurality of capacitors to an average voltage of said second node. 